Hermetic secondary batteries which can be charged and discharged have high energy density. When an internal or external short circuit occurs, or the battery experiences abnormal heat generation or impact, charge/discharge reaction or chemical reaction occurs rapidly in the battery to cause rapid gas generation. This may expand or break a battery case. For this reason, most of the hermetic batteries are provided with a safety valve (an explosion-proof mechanism) through which the gas generated in the battery is discharged outside the battery when a pressure in the battery reaches a predetermined value.
As the safety valve provided in the hermetic battery, a sealing plate including a valve element which breaks when the pressure in the battery reaches the predetermined value may be provided to hermetically seal an opening of a battery case, or a thin part may be formed in part of the hermetic battery case. In the former safety valve, the valve element breaks when the pressure in the battery reaches the predetermined value, thereby discharging the gas generated in the battery outside the battery through a vent hole formed in the sealing plate. In the latter safety valve, the thin part of the battery case breaks when the pressure in the battery reaches the predetermined value, thereby discharging the gas generated in the battery outside the battery through the broken part of the battery case.
In the former safety valve, an area of the vent hole formed in the sealing plate cannot be sufficiently large. Thus, when the gas is generated faster than the gas is discharged, the safety valve cannot function well, and the pressure in the battery may increase to expand or break the battery case. In the latter safety valve, a pressure at which the thin part breaks, i.e., an operating pressure of the safety valve, has to be set high to ensure strength of the battery case. Thus, when the pressure in the battery has increased due to an abnormal event, it takes long time until the safety valve is operated, for which high temperature gas is generated to increase a temperature of the battery. This may thermally damage a device in which the battery is inserted.
In view of such disadvantages, Patent Document 1 describes a hermetic battery including a safety valve formed with a sealing plate including a valve element, and a safety valve formed with a battery case including a thin part. According to the technology of Patent Document 1, a pressure at which the thin part breaks is higher than a pressure at which the valve element breaks. Thus, when the gas is generated slowly, the generated gas can easily be discharged by breaking the valve element only, thereby reducing the increase in battery temperature. When the gas is generated rapidly, the thin part of the battery case breaks, thereby quickly discharging the gas, and preventing the break of the battery case.